As a magnetic recording mode used in commercially available HDDs, the so-called perpendicular magnetic recording mode is the mainstream in recent years.
A granular-type recording layer has a granular structure in which the magnetic crystal grains are surrounded by grain boundary regions made of non-magnetic materials, and therein, the magnetic crystal grains are two-dimensionally and physically isolated from each other by the non-magnetic grain boundary region and such a structure achieves reduction in magnetic exchange interaction between the magnetic grains. Thus, transition noise in recording and reproduction characteristics can be reduced and a limit bit size can also be reduced also. Conversely, since the magnetic exchange interaction between the magnetic grains is reduced in the granular-type recording layer, the distribution of the inverse magnetic field tends to increase depending on the distribution of components and sizes of the grains and such a structure causes increase of the transition noise and jitter noise in the recording and reproduction characteristics.
Furthermore, the lower limit value of the recording bit size is set depending largely on the size of the magnetic crystal grains of the granular-type recording layer. Thus, to increase the recording density of the HDD, the grains in the granular-type recording layer must be fine. As a method for making the grains in the granular-type recording layer fine, a technique is known of using an underlayer having fine crystal grains to make the grains of the granular-type recording layer stacked thereon fine. Here, to make the grains of the underlayer fine, there are several possible methods such as manipulating the seed layer and making the underlayer granular.